1. Field
This invention relates to integrated optical components and, more specifically, to integrated optical fiber to waveguide couplers.
2. Description of Related Technology
Integrated optical devices (e.g., photonic integrated circuits) are well suited to applications in various technologies such as telecommunications, instrumentation, signal processing and sensors. In operation, photonic integrated circuits use optical waveguides to implement devices, such as optical switches, optical couplers and wavelength multiplexers/demultiplexers, for example. Such waveguides, when integrated with a photonic integrated circuit, are typically implemented as solid dielectric light conductors, which are fabricated on a substrate in a very similar fashion as semiconductor integrated circuits are manufactured. Waveguides transmit light around optical circuits and also connect to external optical waveguides, such as optical fibers, typically by direct physical abutment of the fiber with the waveguide. However, in such a configuration, a mode mismatch results between the integrated waveguide and the optical fiber. Specifically, because the difference of the refractive index between the core and cladding of a typical waveguide is higher than that of a typical optical fiber, the optical field is more confined in the waveguide than in the fiber.
In addition, waveguide core dimensions are typically smaller than the core dimensions of the optical fiber. Therefore, such directly physically coupled devices typically realize a 7-10 dB insertion loss in optical signal strength. Moreover, for high index contrast waveguides that are used for dense integrated optics, the insertion loss may be as high as 23 dB because of the even smaller dimensions. In this context, the insertion loss of an optical coupler is the difference (or may be expressed as a ratio) of output to input light power at a particular wavelength. Therefore, coupling a cylindrical optical beam from a fiber into a dielectric (essentially planar and rectangular) waveguide structure and back is a challenge and alternative techniques are desirable.
One non-integrated (not part of the waveguide) solution that has been implemented to address the above concerns is the use of lensed fibers. While such lensed fibers can enhance the coupling efficiency from the fiber to the waveguide, such an approach requires very critical and difficult sub-micron alignment.
An integrated solution that has been employed is the use of spot-size converters. In this regard, lateral, in-plane spot-size conversion can be realized with tapered ridge waveguides. However, vertical spot-size conversion is more difficult and requires more complicated structures. For example, one solution for the vertical spot-size conversion problem is the use of a grating coupler to couple light from an out-of-plane optical fiber to a planar waveguide. Such grating couplers using 1-dimensional gratings (grooves) have been extensively studied, both theoretically and experimentally, see R. M. Emmons, D. G. Hall, “Buried-oxide silicon-on-insulator structures: waveguide grating couplers”, IEEE J. Quantum Electron., vol. 28., pp164-175, January 1992; and D. Taillaert, W. Bogaerts, P. Bienstman, T. F. Krauss, P. Van Daele, I. Moerman, S. Verstuyft, K. De Mesel, R. Baets, “An out-of-plane grating coupler for efficient butt-coupling between compact planar waveguides and single-mode fibres”, IEEE J. Quantum Electron., vol. 38, pp. 949-955, July 2002.
However, existing grating couplers such as described in “Highly directional grating outcouplers with tailorable radiation characteristics” by N. Eriksson, M. Hagberg, A. Larsson, IEEE J. Quantum Electron., vol. 32, no. 6, 1996, p. 1038-1047, have a narrow bandwidth, use a long grating (>100 μm), and work for only one polarization. Therefore, due to these limitations such couplers are not particularly well suited for coupling optical fiber to photonic integrated circuits. A coupler with improved bandwidth and shorter coupling length is disclosed in “A high-efficiency out-of-plane fiber coupler for coupling to high index contrast waveguides”, D. Taillaert, W. Bogaerts, P. Bienstman, T. Krauss, I. Moerman, P. Van Daele, R. Baets, Proceedings 27th European Conference on Optical Communication, 30 Sep. -4 Oct. 2001, Amsterdam, The Netherlands, post-deadline paper Th.F.1.4, p. 30-31. This coupler uses a high refractive index contrast periodic structure, also known as photonic crystal, instead of a long grating.
A second issue presented by coupling optical fiber to most photonic integrated circuits is the polarization dependence of those photonic integrated circuits. Because the state of polarization of light in a standard single mode fiber can change, without otherwise accounting for such polarization changes, photonic integrated circuits to which the fiber is coupled would need to operate independent of polarization of the light communicated from the fiber. However, making a large-scale photonic integrated circuit polarization independent is difficult and for some optical functions is not possible. One possible solution to this problem is to use polarization splitter. Such devices, an example of which is described in European Patent 0738907, split the two polarizations into two different waveguides. This solution, however, has the drawback that two different waveguides and two circuits are needed for the two different polarizations. While a polarization converter would solve this problem, these devices are relatively large (as compared to photonic integrated circuits) and, therefore, not well suited for use with integrated optics. Thus, other techniques for implementing optical couplers are desirable.